The discussion that follows uses cam shaft phaser 100 in FIGS. 1 through 5 as an example. Pin 120 is used to lock rotor 104 to stator 102 for a locked mode for phaser 100 as further described below. To transition from the locked mode to an unlocked mode for phaser 100, fluid F, for example oil, flows to chamber 114A and through channel 126 to displace pin 120 out of indentation 124. If fluid F flows too quickly to chamber 114A: rotor 104 urges pin 120 in direction CD1 before pin 120 has disengaged from cover 106 to jam, or wedge, pin 120 against cover 106; and rotor 104 is unable to rotate to a desired unlocked position for the unlocked mode.
As is known in the art, control valve CV includes one or more electrical elements, such as solenoids, that are energized to control flow of fluid to chambers 114 and 116. The force generated by the electrical elements determines the flow of fluid F to chambers 114 and 116. The force generated by the electrical elements is dependent on the current applied to valve CV and the current subsequently flowing through the electrical elements. The current is dependent upon the resistance of the material forming the elements, for example copper coils, and the voltage applied to the elements, as shown by Ohm's law: I (current)=V (voltage)/R (resistance). Voltage is typically controlled with the use of pulse width modulation (PWM). Resistance of the material is temperature dependent. For example, as temperature of the material increases, so does the resistance. For example, for copper, a temperature difference of 50° C. results in a 20% change in R. Therefore, the function of the solenoids and the flow of fluid F is temperature dependent.
FIG. 11A is a graph of fluid flow versus electrical current for a known method of operating a known cam shaft phaser with an axially displaceable locking pin. FIG. 11B is a graph of pulse width modulation (PWM) voltage versus electrical current for the cam shaft phaser of FIG. 11A. As is known in the art, in FIG. 11A, at zero current, fluid flow is at a maximum to chambers 116 through channels 132. As the current increases, fluid flow to chambers 116 is decreased and the flow is substantially terminated at electric current level 602. As the current level is increased beyond level 602, fluid flow begins to flow to chambers 114. The ideal flow rate of fluid F to chamber 114A occurs at current level 604 and point 606 on oil flow curve 608. That is, for level 604, flow rate 610 for fluid F is enough to flow fluid F from chamber 114A to slot 118 through channel 126 and displace pin 120 out of indentation 124. That is, flow rate 610 does not urge rotor 104 in direction CD1 with sufficient force to jam pin 120 against cover 106 and prevent pin 120 from displacing out of indentation 124.
FIG. 11B illustrates the temperature dependency of point 606. Line 702 is for a first ambient temperature of the material, described above, for the electrical elements. PWM voltage level 704 is needed to generated ideal current level 604. Line 706 is for a second ambient temperature of the material, described above, for the electrical elements. The second temperature is greater than the first temperature; therefore, PWM voltage level 708, greater than voltage level 704, is needed to generated ideal current level 604. PMW voltage is the only input to control valve CV. As further described below, known methods of operating a cam shaft phaser, such as phaser 100, involve the use of a same PWM level regardless of ambient temperature and these methods are not effective at all the ambient temperatures that can be expected for control valve CV.
FIG. 12A is a graph of the duty cycle of PWM voltage versus time for a known method of operating a known cam shaft phaser with an axially displaceable locking pin. FIG. 12A is a graph of measured angle versus time for the known method of operating the known cam shaft phaser of FIG. 12A. For the known method associated with FIGS. 12A and 12B, At time t9, controller C activates power supply PS to transmit PWM voltage, as a rectangular wave form, to control valve CV and initiate the unlocked mode. In FIGS. 12A and 12B, application of the rectangular wave between times t9 and t10 fails to rotate rotor 104 (pin 120 jammed against cover) to the desired unlocked position, for example due to the ambient temperature of control valve CV. For example, rotor 104 has been urged in direction CD1 with sufficient force to jam pin 120 against cover 106 before pin 120 has displaced out of indentation 124. Starting at time t10, a strategy to pulse fluid flow to chamber 114A and slot 118 using a rectangular PWM wave form with duty cycle 802 is employed. The goal of the strategy is break the contact of pin 120 with cover 106 and enable pin 120 to disengage from cover 106. However, the strategy relies on the same duty cycle 802, regardless of temperature, and so is subject to the temperature limitations noted above. Eventually, by time t11, the strategy may be successful. If the strategy is successful, the time span between times t10 and t11 depends on the difference between the actual ambient temperature and the ambient temperature assumed for the pulsing strategy.